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memphissound 11th December 2006 08:20 PM

bypassing the plate-load
 
I was looking at a couple of older guitar amp schematics recently and noticed that in a couple of instances there was a bypass cap on the plate load resistor. Found it in the pre-amp section on some 12AX7's.

Why is this done? How is it's value determined?

Thanks.

peace,
memphissound <><

Joel 11th December 2006 10:32 PM

It's rolling off the gain of the amplifier at high frequencies, probably to control instability due to a feedback loop.

Joel

memphissound 12th December 2006 12:57 PM

I suppose that's in parallel to the load resistor at a value of

1
------------------
2*pi*freq*C

peace,
memphissound <><

EC8010 12th December 2006 01:46 PM

No, you have to find it by experiment (testing with a 10kHz square wave for minimum ringing but maximum sharpness). It's determined by the output transformer and the various HF time constants in the driver circuitry.


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